Exemplary embodiments of the present invention relate to a device and a method for detection of falsified signals in a satellite navigation system, as well as a usage system for signals of a satellite navigation system.
In order to determine a position using a satellite navigation systems users rely on correct signals from the satellites (navigation signals). Correct signals contain the required and correct navigation messages, in particular those having integrity containing the ephemeris of satellites and transmission times as required for determining positions.
In satellite navigation systems currently in use, such as NAVSTAR GPS, satellite signals for civilian users are also transmitted without encryption, and thus it is possible to generate manipulated navigation signals near the ground. For example, falsified navigation signals can be generated and transmitted using pseudolites to prevent a correct position determination with such navigation systems. The manipulated navigation signals here are superimposed on the true signals emitted by the satellites and may contain formally valid but false navigation messages, i.e., falsified information for determination of position, i.e., location.
Transmitting falsified (and/or manipulated) navigation signals is known as “spoofing” because users often cannot differentiate the falsified signals from the true signals and thus correct navigation signals may be simulated for a receiver.
The following methods are known in particular as countervailing measures to spoofing (anti-spoofing):                Authentication of the navigation message: this type of authentication relates only to the navigation message but not to the signal. However, spoofers may provide their navigation signals with the true navigation message and thus calculate the pseudoranges accordingly.        Use of RAIM (receiver autonomous integrity monitoring) techniques in receivers of navigation signals (receiver-integrated logic): this makes it possible to detect one or more classified navigation signals but it does not enable detection of falsification of signals of a complete satellite constellation.        Observation of signal strength in receivers to identify spoofers based on a higher or fluctuating signal strength. This, however, is not very reliable because of the range of fluctuation in received signal strength that occurs even with true signals. Furthermore, it is not very difficult technologically to emit falsified navigation signals in a power range corresponding approximately to the power range of received satellite signals.        Use of directional antennas or multiple antennas with a known geometry, to detect the direction from which the navigation signals are emitted. For example, it is possible to use antennas that receive signals only from satellites above the receiver and suppress signals coming from near the ground. However, this requires an appropriate alignment of a receiver for navigation signals, so that the directional antennas are aimed in the direction of the satellites and not in the direction of the ground.        
As explained above with regard to the known anti-spoofing measures, these measures have some rather great disadvantages in the detection of falsified signals. Accordingly, exemplary embodiments of the present invention are directed to an improved method of detection of falsified signals for a satellite navigation system.
In order to improve the detection of falsified signals in a satellite navigation system, the present invention involves receiving signals of a satellite navigation system with at least two antennas and one or more receivers and determining the phase differences of the received signals for each antenna. In the case of signals from satellites, the phase difference measurements on the individual antennas will yield different phase differences for the same satellite pairs. However, if the signals are emitted by a single source, then the phase differences between the individual signals at each antenna are low, in particular being almost identical. Thus signals emitted by a source can be detected. It is thus possible to detect an individual spoofing transmitter that is emitting multiple falsified navigation signals in a relatively reliable manner. The invention also does not require the technologically relatively complex determination of directions from which the navigation signals are coming by way of directional antennas. Instead, according to the present invention it sufficient to work on the basis of phase differences on the individual antennas, such that, for example, simple omnidirectional antennas may be used as the antennas. Furthermore, neither the distance nor the location of the individual antennas in relation to one another need be constant. Another advantage of the invention is that conventional commercial antennas and receivers for navigation signals that are suitable for determining phase differences may be used.
One embodiment of the invention relates to a device for detection of falsified signals of a satellite navigation system, comprising
at least two antennas having one or more receivers for receiving navigation signals,
a phase difference measuring unit for determining phase differences of the received navigation signals for each antenna, and
a detection unit for detecting falsified navigation signals among the received navigation signals on the basis of the phase differences detected.
Such a device may be at least partially integrated into chip sets for receivers for satellite navigation signals, for example, to be able to efficiently detect falsified navigation signals. The phase difference measuring unit may be integrated into the high frequency unit (RF stage), for example, and the detection unit may be integrated into the signal processor of a chip or chip sets for GNSS receivers.
The phase difference measuring unit may be designed to receive phase differences between the navigation signals received by each antenna based on carrier phase measurements.
Furthermore, a reference wave unit may be provided to generate a reference wave at the same frequency as the carrier wave of the received navigation signals, and the phase difference measuring unit may be designed to determine phase differences between the navigation signals received by each antenna on the basis of measurement of the phase differences between the carrier phases of individual navigation signals and the phase of the reference wave.
The detection unit may be designed to detect navigation signals as falsified navigation signals when the phase differences of these navigation signals are almost identical on each antenna.
The at least two antennas may be omnidirectional antennas.
Another embodiment of the invention relates to a usage system for signals of a satellite navigation system, comprising
a device for detection of falsified signals in a satellite navigation system according to the invention, and
a navigation signal processing unit for processing received navigation signals such that navigation signals recognized as falsified by the device are excluded from processing.
With such a usage system it is possible to efficiently prevent falsified navigation signals from interfering with or falsifying the position determination, which is particularly important in safety-critical applications for position determination such as in aircraft navigation.
Finally, one embodiment of the invention relates to a method for detection of falsified signals by a satellite navigation system comprising the following steps:
receiving navigation signals with at least two antennas with one or more receivers,
determining phase differences of the received navigation signals for each antenna with a phase difference measuring unit, and
detecting falsified navigation signals among the received navigation signals on the basis of the phase differences thereby detected by a detection unit.
Such a method may be implemented in a control program of a usage system for navigation signals in a satellite navigation system, for example, to able to detect falsified navigation signals and to exclude them from a position determination. It may also be integrated into navigation software, which is executed on a certain computer platform and is implemented for processing the navigation signals received.
According to this method, phase differences between the navigation signals received by each antenna can be determined by carrier phase measurements.
This method may also involve the following steps:
generating a reference wave at the same frequency as the carrier wave of the received navigation signals by a reference wave unit, and
determining the phase differences between the navigation signals received by each antenna on the basis of measurements of the phase differences between the carrier frequencies of individual navigation signals and the phase of the reference wave.
In addition, this method can involve the following step:
detecting navigation signals as falsified navigation signals by the detection unit when the phase differences of these navigation signals on each antenna are almost identical.
Additional advantages and possible applications of the present invention are derived from the following description in conjunction with the exemplary embodiments depicted in the drawings.
The terms used in the list of reference numerals attached at the end and the respective reference numerals are used in the description, the claims, the abstract and the drawings.